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148 RICHARDSON'S BLUEBOOK OF PROJECTION
long, narrow auditoriums where the projection angle is moderate and the balcony, if any, is low. Under such conditions they are highly efficient.
Reflection Power and Age of Screen
There are many different types of screen surface, some of which have excellent and some comparatively poor powers of reflection and diffusion. (15) The reflection powers of most screens are good when new, but some of them deteriorate quite rapidly. It is impossible to restore any screen to its original value through cleaning. Moreover, reflection power is lessened with each cleaning. Taking these facts into consideration, it is evident that both projectionists and exhibitors should study screen surface characteristics very carefully, basing such study upon (16) (a) reflection power when new and (b) rapidity with which various surfaces will discolor, or absorb and accumulate dirt, (c) probable percentage of original reflection power that may be restored by cleaning, (d) cost of refmishing the surface and (e) angles at which light will be reflected. It is important to select a screen surface that will most efficiently serve the seating space of the auditorium.
(17) The percentage of incident light reflected is a highly variable item. What percentage of light a screen surface will reflect when new and what it will reflect after six months' use are widely divergent facts. (18) Reflection power varies with the character of the surface. Exposure to air has a decided effect and so has the location of the theatre because in some sections the air is laden with impurities which settle on the screen surface.
It is therefore impossible to gauge exactly the loss of reflection power over a period of time. For example, in Buena Vista, Colorado, deterioration would be relatively slow because the mountain air is very pure and clean. In industrial centers, on the other hand, the air is filled with carbon dust and other impurities and screens lose reflection power quite rapidly.